The 1990 amendments to the United States Clean Air Act require major producers of air emissions, such as electrical power plants, to limit the discharge of airborne contaminants emitted during combustion processes. In most steam power plants in operation today, fossil fuels (such as petroleum or coal) are burned in a furnace including a boiler to heat water into steam. The steam drives turbines coupled to a generator to produce electricity. These fossil-fuel-fired furnaces, however, emit highly polluting flue-gas streams into the atmosphere. These flue-gas streams typically contain noxious gaseous chemical compounds, such as carbon dioxide, chlorine, fluorine, NOx, and SOx, as well as particulates, such as fly ash, which is a largely incombustible residue that remains after combustion of the fossil fuel.
To date, many devices have been used to reduce the concentration of contaminants emitted by fossil-fuel-fired furnaces. One of the most effective devices is an electrostatic precipitator (ESP). ESPs and their use in a typical fossil-fuel-fired boiler are described in detail in U.S. Pat. No. 6,488,740. An ESP is a device with evenly spaced static conductors, typically plates, which are electrostatically charged. When a flue-gas stream is passed between the conductors, particulates in the flue gas become charged and are attracted to the conductors. Typically, twenty to sixty conductors are arranged parallel to one another, and the flue-gas stream is passed through passages formed between the conductors. A layer of particulates formed on the conductors limits the strength of the electrostatic field and reduces the performance of the ESP. To maintain performance, the conductors are periodically cleaned to remove the collected particulates.
There are two types of ESPs: dry and wet. A dry ESP removes particulates from the conductors by shaking or rapping the conductors and collecting the removed particulates in a dry hopper. A wet ESP removes the particulates by washing the particulates off the conductors and collecting the removed particulates in a wet hopper.
A system for removing particulates using a series of dry ESP fields and a wet ESP field is disclosed in U.S. Pat. No. 3,444,668. This system removes particulates in a cement manufacturing process. However, positioning a wet ESP field upstream of a dry ESP field, such as that disclosed in U.S. Pat. No. 2,874,802, does not sufficiently remove contaminants from a flue-gas stream or address the above-described problems.
U.S. Pat. Nos. 5,384,343 and 5,171,781 disclose a process of pelleting coal fines with superabsorbent fines that have been aggregated for use in fossil-fuel furnaces including the steps of converting a wet sticky mass of coal fines to a crumbly or flowable solid and then pelleting the solid. The '343 and '781 patents disclose making the wet, sticky mass of coal fines with water absorbent polymer particles that are fines, particle size of less than 10 μm, that are selected from starch acrylonitrile graft copolymers and polymers formed by polymerization of water soluble ethylenically unsaturated monomer or monomer blend. In particular, the polymer particle fines have an effective dry size of less than 10 μm. The fines are then aggregated, and the aggregate polymer is made up of a mixture of superabsorbent polymers of at least 90% below 50 μm and are mixed into the mass of particulate material, while the particles are in the form either of a dry powder having a particle size above 50 μm and which consists of internally bonded friable aggregates of finer particles below 50 μm in size, or of a dispersion of particles below 50 μm in size in water immiscible liquid. In essence, the '343 and '781 patents are directed to the use of superabsorbent polymer fines, which are aggregated and used to pelletize combustion fuel such as coal.
The '343 and '781 patents further teach that the use of absorbent particles as low as 50 μm or less is therefore generally undesirable, but a tendency with the use of larger particles, e.g., 200 μm and above, is that their rate of absorption of liquid from the environment can be rather slow and, if such particles aggregate, then the aggregates are rather large, and this can be undesirable.
In view of the foregoing, it would be highly desirable to provide a fossil-fuel-fired system including an efficient system for decreasing the concentration of contaminants within a flue gas emitted by a fossil-fuel-fired furnace, while addressing the above described shortfalls of prior art systems.